1. Field of the Invention
The present invention relates generally to a mobile communication system supporting an enhanced uplink dedicated channel (hereinafter referred to as “EUDCH”) service, and in particular, to a method for determining a data rate for an EUDCH service of a user equipment (UE) by a Node B in performing control scheduling on the UE.
2. Description of the Related Art
The present invention is provided on the assumption that an enhanced uplink dedicated channel (EUDCH) is used in a wideband code division multiple access (WCDMA) communication system. The EUDCH is a channel proposed to improve packet transmission capability for uplink transmission in an asynchronous code division multiple access (CDMA) communication system. For the EUDCH technology, new short transmission time interval (TTI) technology can be used together with AMC (Adaptive Modulation and Coding) and HARQ (Hybrid Automatic Retransmission Request) used in existing HSDPA (High Speed Downlink Packet Access). The TTI can be defined as a transmission unit for which one data block is transmitted over a physical channel. In HSDPA, as scheduling of a downlink channel is performed not by a radio network controller (RNC) but by a Node B, scheduling of an uplink channel is also performed by the Node B. Of course, Node B's uplink control scheduling is greatly different from Node B's downlink control scheduling.
FIG. 1 is a fundamental conceptual diagram illustrating a situation where EUDCH is used. In FIG. 1, reference numeral 100 represents a Node B supporting EUDCH, and reference numerals 101 to 104 represent user equipments (UEs) transmitting EUDCH. The Node B 100 analyzes channel conditions of UEs that use the EUDCH, and performs proper scheduling on each UE. The scheduling is performed in such a manner that a low data rate is assigned to a UE located far from the Node B and a high data rate is assigned to a UE located close to the Node B as long as a measured noise rise value of the Node B does not exceed a target noise rise value, in order to increase the entire system capability.
A fundamental EUDCH transmission/reception procedure will be described with reference to FIG. 2. FIG. 2 illustrates a transmission/reception procedure between a UE 202 transmitting EUDCH and a Node B 201 to which the UE 202 belongs. The Node B 201 and the UE 202 perform EUDCH transmission/reception setup in step 203. The setup process includes a process of delivering messages over a dedicated transport channel. After EUDCH setup is performed in step 203, the UE 202 sends scheduling information to the Node B 201 in step 204. The scheduling information sent in step 204 can include the UE's transmission power information from which uplink channel information can be detected, the UE's transmission power margin information, or an amount of transmission data stored in a buffer of the UE. The Node B 201, receiving the scheduling information from a plurality of UEs, schedules the respective UEs while monitoring the scheduling information from the UEs in step 211. A scheduling method can be dependent upon the Node B 201, and a detailed description of the scheduling method will now be described.
When the Node B 201 schedules the UE 202 according to the process of step 211, the Node B 201 transmits scheduling assignment information to the UE 202 in step 205. The UE 20 then transmits EUDCH using an assigned data rate and transmission timing included in the scheduling assignment information of step 205 in step 207. A transport format resource indicator (hereinafter referred to as “TFRI”), which is resource information of EUDCH of step 207, is transmitted to the Node B 201 together with EUDCH of step 207, in step 206. After receiving the channels of steps 206 and 207, the Node B 201 determines whether there is an error in the TFRI of step 206 and the EUDCH of step 207. If there is any error in either the TFRI of step 206 or the EUDCH of step 207, the Node B 201 transmits NACK information to the UE 202 over an ACK/NACK channel in step 208. However, if there is no error in both the TFRI of step 206 and the EUDCH of step 207, the Node B 201 transmits ACK information to the UE 202 over the ACK/NACK channel in step 208.
Meanwhile, the Node B 201 determines a data rate to be designated to the UE based on the scheduling information received in step 204. In this process, the Node B must assign a proper data rate and transmission timing to several UEs using EUDCH, and resources must be assigned to the UEs so that an uplink noise rise value should not exceed a target noise rise value in the scheduling. Of course, for improvement of the entire system capability, more resources are assigned to a UE having a better channel condition.
Herein, a description will be made of a procedure for scheduling a UE by a Node B in transmitting and receiving EUDCH. As described above, the Node B schedules EUDCH transmission of several UEs so that a noise rise value should not exceed a target noise rise value, and at the same time, the Node B's capacity should be maximized. The Node B performs such scheduling using the scheduling information received from respective UEs in step 204. The scheduling information of step 204 can be used in the following two methods.
In a first method, each UE notifies a Node B of its transmission power value. Also, the UE can inform the Node B of an amount (quantity size) of data stored in its buffer. In this method, the Node B can estimate an uplink channel condition in a situation that each UE faces, using transmission power of the UE, so it can assign proper resources to each UE.
A detailed description of the method will now be made with reference to FIG. 1. In FIG. 1, the UEs 101 to 104 are different from each other in distance from the Node B 100, and the UE 101 is located closest to the Node B 101 while the UE 104 is located farthest from the Node B 101. In this case, the UE 101 has the lowest uplink channel power strength (represented by a thinnest arrow 111), and the UE 104 has the highest uplink channel power strength (represented by a thickest arrow 114). Therefore, as a method for obtaining the highest capability while maintaining the same measured noise rise value, scheduling is performed in such a manner that power strength should be in reverse proportion to a data rate. That is, scheduling is performed in such a manner that a UE located close to a Node B, like the UE 101, having low uplink transmission power is assigned the highest data rate, while a UE located far from the Node B, like the UE 104, having high uplink transmission power is assigned the lowest data rate. Such a method is called “maximum CQI (Channel Quality Indicator) scheduling.” However, in this method, the Node B has no information on a transmission power margin available for each UE, increasing possibility that flexibility of scheduling will be lost.
That is, even though more resources are assigned to a UE having a good uplink channel environment, if a transmission power margin of the UE is not sufficient, the UE cannot sufficiently use the assigned resources. For example, since the UE is located close to the Node B 100, like the UE 101, it can transmit data at low uplink transmission power. In addition, although the UE can be assigned a relatively high data rate in transmitting data, if a transmission power margin of the UE is not sufficient, the UE, in some cases, cannot use maximum resources determined by the Node B 100. That is, as described above, since the Node B 100 does not have information on an available power margin of the UE 101, the Node B 100 cannot effectively determine how many resources it should assign to the UE 101.
In a second method, a transmission power margin of a UE is determined with the scheduling information. A UE informs a Node B of its available power margin, and the Node B receiving the transmission power margins from several UEs assigns resources to the UEs through scheduling so as to efficiently increase cell capability.
However, in this method, the Node B cannot accurately detect a channel condition of each UE. That is, transmission power margin information that the UE sends to the Node B does not have uplink channel condition information of the UE. Therefore, the maximum CQI scheduling method that performs scheduling according to a channel condition cannot be used.
For example, according to this method, when a transmission power margin is transmitted from the UEs 101 to 104 to the Node B 100, relatively many resources are assigned to a UE having a large power margin while relatively fewer resources are assigned to a UE having a small power margin. In this case, even the UE having a large power margin, when its channel environment is poor, cannot be sufficiently assigned as many resources as the value for which the power margin is considered. That is, even though sufficient resources as determined by the power margin are assigned, normal data transmission/reception becomes difficult due to the poor channel environment, causing a reduction in channel capacity.
As described above, a Node B assigns resources to UEs that use EUDCH, through scheduling. The scheduling is performed using scheduling information delivered by a UE over an uplink. The above-stated two proposed conventional methods lack information for optimized scheduling. Accordingly, there is a demand for a method for maximizing system capability by efficiently performing scheduling in which the Node B assigns resources to UEs that use EUDCH.